Blog-2: Network Infrastructure & Call Basics

RIL acquired 95% stake in IBSL soon after the latter acquired a pan India 4G license (22 circles). IBSL had bought the spectrum for 12,750cr way back in June 2010. If we consider a risk free interest rate of 7.5%, RIL is losing on an opportunity cost of close to whopping 2.5 crores a day, forget the fact that funds may be raised through debt. On top of this RIL will have to follow a minimum roll-out obligation of 90% of service area within five years of acquiring spectrum which turns out to be Aug 2015. The vision of Digital India proposed by Sh Modi is also heavily dependent on how quickly and with what stability the network is rolled out. Traditionally RIL has launched big initiatives on Sh Dhirubhai’s birthday (28th Dec) but it seems too late this year. We speculate a Diwali launch.

In my last write-up, I covered briefly the history of telecom in India. We discussed different generations of mobile telephony in detail. I ended with a brief comparison between 4G and the rest of the technologies. In this write up we will continue with the comparison but a little later. First we will look at what constitutes a telecom network both from an infrastructure and architecture point of view. We will discuss the architectural similarities between 2G and 4G network and conclude by understanding the mechanism of a phone call in a 2G network.

Naksha

Before I write at depth of how calls are made and the kind of backend infrastructure needed to support it, I would like you to think in rudimentary terms how can we speak to a person sitting far away from us. Once you can visualize that, involve the component of simultaneity (multiple parties making calls together). As you would have guessed by now, we must have some sort of wireless communication protocol between mobile handsets to communicate. If our mobile handsets were to be made powerful enough to communicate directly to another handset, it would emit a lot of radiation thus making it non user friendly. So we have helpers in between which make the connection possible. Imagine multiple helpers in between two mobile phones guiding the messages safely from end to end. To accommodate simultaneity, the helpers must be able to work like postmen – direct the calls to the addressee.

From an infrastructure stand point there are three components in a telecom network –

  • Towers/ Masts
  • Optical Fibers
  • Facilities

The big steel skeletal structures which you see on ground and sometimes on rooftops are the mobile towers. They will contain some basic telecom equipment to serve the mobile phones in the nearby area. The area close to a mobile tower is usually prone to a higher dose of radiation. The equipment on the towers are connected to other mobile towers or central facilities. These facilities have information processing capabilities. They can monitor towers, provide diagnostics, route calls and many more things. These interconnections are done through a vast network of optical fibers. Optical fibers are very thin glass tubes which provide a passage for light to pass through. Yes, in a telecom network more often than not, messages are transmitted as light. In a separate section I will describe the construction aspect of this mammoth structure.

All the mobile generations have survived on this basic infrastructure. With the evolution of 3G and 4G, the equipment have become more robust and powerful. The difference between these generations can be clearly understood from an architecture point of view. However I am afraid the discussion may become too technical. Still a basic overview of 2G architecture is given below.

In a GSM (2G) network, there are 4 basic components –

  • Mobile Station (MS)
  • Base Station Subsystem (BSS)
  • Network Switching Subsystem (NSS)
  • Operation Support Subsystem (OSS)

Mobile station comprises of 2 items – Mobile handset and the SIM card. SIM card is a chip which contains user specific data (subscribed mobile services). Mobile handset is the device which is user agnostic. It helps in modulating and encrypting the voice/ data ready to be sent to helpers.

Base Station Subsystem comprises of 2 items – Base Transceiver Station (BTS) and Base Station Controller (BSC). The next time you see a mobile tower, look at the tall vertical loudspeaker type devices attached to the tower. In all probability you are looking at a BTS. BTS is a helper. A BTS covers a cell (a small geographical area of network coverage). The Mobile handset transmits the voice/ data to the BTS. Now think of BSC as a manager of BTS. It monitors performance of multiple BTS and does higher order functions such as assigning time slots for Mobile handsets (kind of a postman). In short, if multiple callers are calling in the same cell, BSC manages their transmission ensuring that all calls go through with minimal losses.

Network Switching Subsystems consists of multiple entities, most important of which are Mobile Services Switching Center (MSC), Home Location Register (HLR), Visitor Location Register (VLR) and Authentication Center (AuC). MSC is responsible for switching calls between mobiles and/ or fixed lines. HLR is a directory containing information of every mobile registered in the area managed by the MSC. VLR is a directory containing temporary information of visiting subscribers (roaming numbers). AuC helps in authenticating and protecting the communications.

Operation Support Subsystem can be thought of as a manager of BSS and NSS. It provides an overview of the network and supports the maintenance of various systems.

Information overload? Unfortunately this is how it is. The complexity is what makes a phone call so simple.

The 4G network (LTE) architecture is not much different from GSM architecture. It is comprised of three components –

  • User Equipment
  • Evolved UMTS Terrestrial Radio Access Network (E-UTRAN)
  • Evolved Packet Core

The User Equipment is nothing but the mobile device and the SIM card (also known as UICC – Universal Integrated Circuit Card) analogous to Mobile Station.

The E-UTRAN is a system of Evolved Node-B (analogous to BTS) which handles communication between User Equipment and Evolved Packet Core. It also takes care of handover for mobile equipment.

Evolved Packet Core (analogous to MSC) have various components such as HSS (Home Subscriber Server – similar to HLR), Packet Data Network Gateway which communicates with the outer world, Mobile Management Entity (MME) responsible for signaling and controlling HSS.

Let us move on now and make a phone call…

Classic Alice and Bob

Alice speed dials Bob. The SIM card registered to Alice initiates a call request. The mobile handset with Alice helps in formulating and communicating the request to BSS. The BSS passes on the call request to NSS (more specifically the MSC). The MSC checks if the mobile station/ handset of Alice is authorized to make a call request. Once authorization is complete, MSC instructs BSS to allocate a traffic channel to the mobile handset. This information is also passed on to the mobile station. This completes Alice’s side of call setup. On the other side, MSC forwards the call request to its parent. Through efficient routing, the request reaches the Bob side MSC. When the respective MSC receives a call request, they query the HLR to route the call to Bob’s mobile phone. Once a route is established, a broadcast message is sent to Bob’s mobile device which responds and acknowledges. This completes a circuit (a dedicated path from one mobile to another). Then the concerned BSC allocates a traffic channel and once the subscriber answers the call, the connection is established.

If nothing I wrote made any sense, forget all I said and I’ll try to explain it in a different manner.

In a telecom network all you have can be categorized into two basic entities; intelligent devices and dumb devices. The intelligent devices in the network guide the messages (voice/ data) in the right direction. The dumb devices provide a pathway for the messages to flow. For simplicity we will assume the intelligent devices only consist of routers and dumb devices consist of optical fibers and network towers.

When we make a phone call, the nearby tower picks up the signal from the mobile. The voice/ data is sent from the tower to the nearest facility. Remember facility being the third component of telecom infrastructure. Think of this facility as an aggregator of multiple calls. This is where it can handle multiple calls and route them effectively. Now this first level aggregation is nearly not enough to handle the capacity of Indian mobile market. There are second level and third level aggregators. They are bigger and more robust. So the call goes from a mobile device to the nearby tower to the nearby facility. The facility forwards it to the nearest super facility and perhaps then a national facility. You may get the picture now. From there the call goes through levels of disaggregation to reach the mobile device to which we are calling.

I hope this has given an insight into the telecom infrastructure and call basics. In the next piece, I will talk about building up telecom infrastructure from scratch.

http://www.tutorialspoint.com/lte/lte_network_architecture.htm

http://www.tutorialspoint.com/gsm/index.htm

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